Newton’s First Law.

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2 Newton’s First LawNewton’s First Law of Motion States: an object at rest remains at rest, and an object in motion continues in motion with constant velocity (constant speed in a straight line) unless it experiences a net external force.

3 Newton’s Fist LawThe net external force is the total force resulting from a combination of external forces on an object.

9 PracticeJoe ponder, from North Carolina, once used his teeth to lift a pumpkin with a mass of 275 kg. Suppose Ponder has a mass of 75 kg, and he stands with each foot on a platform and lifts the pumpkin with an attached rope. If he holds the pumpkin above the ground between the platforms, what is the force exerted on his feet?Fnet= (9.81)(275+75)Fnet= up

10 PracticeThe net force exerted by a woodpecker’s head when it strikes the bark of a tree can be as large as N, assuming that the bird’s head has a mass of 50.0 g. Assume that two different muscles pull the woodpecker’s head forward and downward, exerting a net force of 4.90 N. If the forces exerted by the muscles are at right angles to each other and the muscle that pulls the woodpecker’s head downward exerts a force of 1.70 N, what is the force exerted by the other muscle? Be sure to draw a free-diagram showing the forces acting on the woodpecker’s head.Fnet²=(Fx)² +(Fy)²4.90N²= Fx² +(1.70N)²Fx= 4.59 N